The present invention relates to a method for manufacturing a superconductor having a superconductive intermetallic compound of at least two elements.
In particular, the present invention relates to a method for manufacturing a superconductor having a superconductive intermetallic compound of at least two elements in which an intermediate conductor product is prepared from a first component containing one element of the compound and a second component which is an alloy consisting of a carrier metal and the remaining element or elements of the compound, and in which the intermediate conductor product is subjected to a heat treatment such that the desired intermetallic compound is formed by reaction of the element of the first component with the remaining element or elements of the second component. Such a manufacturing method for a superconductor is known from DE-OS No. 2 056 779.
Superconductive intermetallic compounds consisting of two components, each having one element of the desired intermetallic compound, such as Nb.sub.3 Sn or V.sub.3 Ga, which are of the A.sub.3 B type and have A15 crystal structure, have very good superconductor properties and are distinguished particularly by a high critical flux density B.sub.c2 in a magnetic field, a high transition temperature T.sub.c and a high critical current density I.sub.c. They are, therefore, particularly well suited as conductors for superconductor coils for producing strong magnetic fields. In addition, ternary compounds such as niobium-aluminum-germanium Nb.sub.3 (A1.sub.x Ge.sub.(1-x)), are of special interest. Since these compounds are generally very brittle, however, it is very difficult to produce them in a form suitable, for example, for magnet coils. From the mentioned DE-OS No. 2 056 779, a method is known which makes possible the manufacture of superconductors with intermetallic compounds of two components in the form of long wires or ribbons. This method serves in particular for the manufacture of so-called multicore conductors with wires arranged in a normal-conducting matrix, for example, of Nb.sub.3 Sn or V.sub.3 Ga or with niobium or vanadium wires with surface layers of these compounds. Thus, a ductile element of the compound to be produced, in wire form, for example, a niobium or a vanadium wire, is surrounded by a jacket of ductile matrix material which contains a predetermined amount of the other elements in the form of an alloy, for example, of a tin or gallium bronze. A mulitiplicity of such wires also can be embedded in the matrix. The structure so obtained is then subjected to a cross section-reducing process and cut into a predetermined number of sections. These sections then are bunched together and again brought into an elongated form through cross section reduction. Through the cross section reductions, the diameter of the wire cores (consisting, for example, of niobium or vanadium) is reduced to a low value in the order of 10 .mu.m or less, which is advantageous in view of the superconductor properties of the conductor. With this process step, a good metallurgical bond is obtained between the wire cores and the matrix material surrounding them without the occurrence of reactions which would embrittle the conductor. Thus, a not yet fully reacted intermediate product of the superconductor in the form of a long wire is obtained such as is subsequently required for the winding of coils. This intermediate product is finally subjected to an annealing treatment in vacuum or in an atmosphere of an inert gas such as argon, where element or elements of the superconductive compound to be formed, contained in the matrix, diffuse into the material of the wire cores, consisting of the other element of the compound, and thus react with the latter, forming a layer consisting of the desired superconductive compound.
It has been attempted repeatedly to increase the current carrying capacity of such multicore conductors by special alloying additives. Thus, small amounts of tantalum have been added to the core material niobium (IEEE Trans. Magnetics, MAG-14, No. 5, September 1978, pages 611 to 613). It is further known to add small amounts of gallium to a bronze matrix (J. Appl. Phys. 49(1), January 1978, pages 357 to 360). While an increase of the critical current density of the superconducting Nb.sub.3 Sn layers can be achieved with these measure, particularly in magnetic fields with flux densities above 10 Tesla, the workability of one or the other conductor components is generally made more difficult by these additions, particularly due to alloy precipitation hardening.